quadrant

n.

The fourth part; the quarter.

The quarter of a circle, or of the circumference of a
circle, an arc of 90¡, or one subtending a right angle at the center.

One of the four parts into which a plane is divided by
the coordinate axes. The upper right-hand part is the first quadrant;
the upper left-hand part the second; the lower left-hand part the
third; and the lower right-hand part the fourth quadrant.

An instrument for measuring altitudes, variously
constructed and mounted for different specific uses in astronomy,
surveying, gunnery, etc., consisting commonly of a graduated arc of
90¡, with an index or vernier, and either plain or telescopic sights,
and usually having a plumb line or spirit level for fixing the vertical
or horizontal direction.